The present invention relates to vibration motors.
Vibration motors are also known as xe2x80x9cultrasoundxe2x80x9d motors which refers to their preferred operating frequency, or as xe2x80x9cpiezoactivexe2x80x9d motors which refers to their preferred excitation material.
The invention is particularly advantageous in application to rotary vibration motors, however it can also be applied to linear actuators, the term xe2x80x9cvibration motorxe2x80x9d in the present specification covering both rotary motors and linear actuators.
A rotary vibration motor conventionally comprises at least one stator and one rotor, together with execution means for deforming said stator and/or said rotor in vibration modes that combine tangential vibrations and normal vibrations suitable for driving the rotor in continuous rotation.
Particularly advantageous vibration motor structures are proposed in particular in patent applications EP 0 907 213 and FR 98/10391 to which reference can advantageously be made. The general principle of those structures is shown diagrammatically in FIGS. 1 and 2.
Such a motor comprises an outer casing 2 containing two rotor disks 1 secured to a shaft 3 together with stator plates 4 between which said disks 1 are disposed.
Each stator plate 4 is constituted by a plurality of contact sectors 6 (referred to as stator xe2x80x9cpetalsxe2x80x9d) which are angularly distributed and which are separated in pairs by active elements 7 for imparting tangential deformation (piezoelectric or other elements). The contact sectors 6 of the two inner plates 4 are in register with one another. Active elements 8 (piezoelectric or other elements) for generating a normal force are interposed between the sectors 6 and these two inner plates 4. Spring-forming means 9 are interposed between the casing 2 and the contact sectors 6 of the outer plates 4.
When an active element 8 lengthens, the contact sectors 6 in register therewith clamp the rotor disks 1. When it retracts, the corresponding contact sectors 6 release the disks 1.
Two active elements 7 on either side of the same sector 6 are excited in phase opposition. Similarly, two adjacent active elements 8 are likewise excited in phase opposition. The active elements 8 for generating a normal force and the active elements 7 for tangential deformation are controlled synchronously so as to drive the rotors 1 in rotation.
One of the problems encountered with vibration motors is that of their efficiency.
Proposals have already been made to use a material having resilient properties in a reciprocating contact zone between a rotor and a stator so as to minimize energy dissipation associated with cyclic friction between the rotor and the stator.
In particular, French patent application FR 2 742 011 proposes using shape memory alloys which are materials having non-linear super-elasticity and which, compared with conventional materials, have the advantage of accommodating large amounts of deformation in small quantities of material.
The term xe2x80x9csuper-elasticityxe2x80x9d is used throughout the present specification to designate the property whereby a material can accept reversible elongation of 1% or more. It is also recalled that the non-linear character of super-elasticity gives rise to the presence of a change-of-phase plateau in the curve of deformation as a function of traction force.
In above-mentioned patent application FR 2 742 011, it is shown that such a structure has the advantage of limiting the peaks of the applied normal force and of maintaining the tangential friction force at values below the slip threshold.
An object of the invention is to further increase the efficiency of vibration motors.
Application BP 0 543 114 discloses an actuator in which bearing contact between the fixed portion and the moving portion is minimized so as to ensure that energy losses due to friction are minimized. In the solution proposed in that document, the contact surface of the fixed part as constituted by the stator is not rigid, but is deformed by the propagation of a traveling wave which drives the moving part which constitutes the rotor. Only the peak of the continuous deformation then comes into contact with the driven part.
It will be understood that that solution does not make it possible to obtain high drive powers of a kind that would be obtainable from vibration motor structures as described above with reference to FIGS. 1 and 2 and operating on the principle of netting rigid sectors into vibration which sectors are moved bodily and not by continuous deformation.
The invention proposes a solution for increasing the efficiency of a vibration motor of the type comprising comprising at least one stationary part and one part driven to move relative to said fixed part, together with excitation means suitable for exerting forces that tend to move rigid contact sectors presented by said fixed part and/or said moving part and to cause said rigid sectors to vibrate in vibration modes that combine tangential vibration and normal vibration, thereby driving the movement of the moving part.
For the tangential vibrations or the normal vibrations, said motor presents a main resonant mode and at least one secondary resonant mode, and the proposed solution consists in that the secondary resonant mode is at a frequency which is substantially equal to a harmonic frequency of the main resonant mode.
In particular, the moving part can be a rigid disk rotor, said motor having a stator which comprises at least one pair of stator plates, each plate having rigid petals suitable for receiving means for displacing said rigid petals tangentially and normally.
In a variant, the motor can be a linear actuator.
In a first advantageous variant, at least one element having elastic deformation properties is included is in the moving part and/or the stationary part, said element being separated from the contact face of said moving part and/or of said fixed part by a shoe-forming portion, and
the part (s) In which the elastic deformation elements are included is/are dimensioned in such a manner that the frequency of the secondary tangential resonant mode which is the resonant mode in which the shoe-forming portion and the remainder of the part oscillate in phase opposition, is substantially equal to a frequency which is a harmonic frequency of the main tangential resonant mode, in which the shoe-forming portion and the remainder of the part oscillate in phase.
Such a motor advantageously further includes the various following characteristics:
the frequency of the secondary tangential resonant mode is substantially equal to twice the frequency of the main tangential resonant mode;
it includes an array of elastic elements interposed between the shoe-forming portion and the remainder of the stationary part and/or the moving part; and
the elastic element is made of a material presenting properties of super-elasticity.
In another, likewise advantageous variant, which can be implemented in addition to the first variant or independently thereof, the motor presents a secondary normal resonant frequency which is substantially a harmonic frequency of the main normal resonant frequency, and the excitation means comprise means for generating normal vibrations at both of these two resonant frequencies.
Such a motor advantageously further includes the various following characteristics taken singly or in any feasible combination:
it comprises a casing containing at least two is pairs of stator plates having tangential deformation active elements, and two rotor disks which extend between the plates of respective ones of said two pairs, the normal deformation active elements extending in particular between the plates of both of the two facing pairs, spring-forming means being interposed between the pairs of plates and the casing, and
it includes, between the stator plates and the spring-forming means, at least one assembly comprising a mass and an elastic deformation element, said assembly being dimensioned in such a manner that the frequency of the secondary resonant mode in which the stator plates and said mass oscillate in phase opposition, is substantially equal to an integer number of times the main resonant frequency in which the stator plates and said mass oscillate in phase, the excitation means including means for exciting normal deformation active elements at a frequency which is substantially equal to the secondary resonant frequency;
an elastic deformation element is a normal deformation active element excited at a frequency substantially equal to the secondary resonant frequency;
a normal deformation active element for the main resonant mode is excited by a signal which is the sum of a signal at the main resonant frequency plus a signal at the secondary resonant frequency;
the secondary resonant frequency is substantially equal to an odd number of times the main resonant frequency;
the secondary resonant frequency is substantially equal to three or five times the main resonant frequency;
the frequency of the secondary resonant mode is equal to an integer number of times the main resonant frequency, with accuracy of the order of {fraction (1/2Q )}where Q is the lower of the quality factors of the two resonances, and
it includes at least one assembly comprising a mass and a plurality of elastic deformation elements interposed between the stator plates and the spring-forming means, the elastic deformation elements being of stiffnesses such that said elements correspond to a plurality of harmonic resonant frequencies.